This invention relates to connecting an electronic component to a copper pad on a printed circuit board or the like utilizing a tin-zinc solder alloy. More particularly, this invention relates to forming such tin-zinc solder connection wherein the copper pad is first coated with a zinc-free tin layer that forms a barrier between the zinc alloy and the copper to prevent dezincification during use that would otherwise compromise the mechanical integrity of the connection.
In the manufacture of a microelectronic package, it is common practice to mount an electronic component onto a printed circuit board or the like utilizing a solder connection that not only physically attaches the component to the board, but also electrically connects the component to a circuit trace on the board for conducting electrical signals to or from the component for processing. The circuit trace is formed of copper to provide high electrical conductivity. The circuit trace includes a bond pad that constitutes a first faying surface for the connection. The component includes a contact or other feature that constitutes the other faying surface. One method for forming the solder connection utilizes a solder paste. A typical paste contains particles of solder alloy dispersed in a vaporizable vehicle that includes flux and an expendable organic binder. The deposit of the paste is applied to the bond pad, after which the component is assembled with the second faying surface in contact with the deposit. The assembly is then heated to melt and reflow the solder alloy. Upon cooling, the solder alloy resolidifies and bonds to the faying surfaces to complete the connection.
In the past, most common solders were based upon tin-lead alloy. Tin and lead form a eutectic composition having a melting temperature of about 183.degree. C., which is suitably low to permit reflow while avoiding thermal damage to the component or other features typically formed on the printed circuit board, but is sufficiently high to withstand temperature excursions of the type experienced by the package during normal operations. However, it is desired to reduce the lead content in electronic packages by forming solder connections of alloys that are essentially free of lead.
It has been proposed to form solder connections to faying surfaces that are aluminum using a tin-zinc solder alloy. However, zinc alloy in the presence of copper undergoes dezincification, a phenomenon by which zinc migrates to the copper interface, leaving voids in the solder that reduce the mechanical integrity of the connection. Also, copper that may dissolve into the solder during reflow may form a galvanic coupling with the zinc and accelerate corrosion of the connection. Difficulty is encountered in reflowing the tin-zinc alloy to wet the copper surface, which wetting is essential to forming a strong solder bond. Thus, zinc solder alloys have not heretofore been utilized in forming solder connections to copper pads on printed circuit boards.